1. Field of the Invention
The present invention relates to an optical device which can periodically or sequentially vary an optical property of the optical device, such as the focal length of a lens, the deflection angle of a prism, the divergence angle of a lenticular lens and so on.
Further, the present invention relates to a three-dimensional display device and its driving method. More specifically, the present invention relates to a technology effectively applicable to an apparatus for displaying a two-dimensional image to be displayed on a two-dimensional display device in a three-dimensional fashion.
2. Description of the Related Art
Most of the conventional optical devices are passive optical devices. The kinds of active optical devices whose optical properties can be varied by voltage or the like are quite limited. Amongst them, as an optical device employing a material having a variable refractive index, there is a liquid crystal lens disclosed in Science Research Expenditure Subsidy Research Results Report No. 59850048 (1984).
FIG. 1 shows the construction of such a liquid crystal lens. The liquid crystal lens having optical properties to be varied by voltage or the like shown in FIG. 1 is constructed with a planar convex lens 1 formed of a polymer, glass or the like, a transparent electrode formed on the surface of the planar concave lens 1, an alignment layer formed of a polyimide or the like on the transparent electrode 2, a liquid crystal 4 (ordinary nematic liquid crystal having an anisotropy of its dielectric constant which is not reversed by difference of frequency), an opposite substrate 5 opposite to these components, a transparent electrode 6 formed on the opposite substrate 5, an alignment layer 7 formed of polyimide or the like on the transparent electrode 6, and a driving device for driving these components. Here, the alignment layers 3 and 7 are in a homogeneous alignment condition for aligning the liquid crystal 4 substantially in parallel.
In the condition where no voltage is applied between the transparent electrodes 2 and 6, the liquid crystal 4 is aligned to be substantially parallel to the alignment layers 3 and 7 by the action of the alignment layers 3 and 7. In this case, an incident light beam 11 that is polarized parallel to the alignment direction is subject to an extraordinary refractive index of the liquid crystal 4. Thus, for example, the liquid crystal 4 appears to have a large refractive index in comparison with the planar concave lens 1 so that the entire optical device serves as a planar convex lens to cause convergence as an output light beam 12.
On the other hand, in the condition where an appropriate voltage is applied between the transparent electrodes 2 and 6, the liquid crystal 4 is aligned to be perpendicular to the electrode 2 and 6. In this case, the incident light beam 11 is subject to the ordinary refraction of the liquid crystal 4. Therefore, for example, the liquid crystal 4 appears to have substantially the same refractive index as the planar concave lens. Then, the entire optical device merely serves as glass plate to output a light beam 13 having substantially the same direction as the incident light beam 11.
Even in such a conventional optical device, it has been possible to sequentially vary an optical property, e.g. focal length, of the planar convex lens depending upon an applied voltage. One example of this relationship is illustrated in FIG. 2.
However, the conventional optical device has the following detects. Alignment of the liquid crystal 4 in the condition where no voltage is applied, is performed only by an anchoring force of the alignment layers 3 and 7. In such a optical device, since the liquid crystal 4 has a large thickness of several hundreds μm or more, a drawback has been encountered in that a resumption timing upon driving is delayed significantly by several seconds, as shown in FIG. 3. Furthermore, even if the applied voltage is increased, the resumption timing can be hardly improved. Therefore, currently, there is no effective method for shortening a resumption period.
As set forth above, when the liquid crystal 4 is aligned only by the anchoring force of the alignment layers 3 and 7, molecules 4a of the liquid crystal 4 may be aligned along a curved surface of the planer concave lens in a portion located in the vicinity of the transparent electrode 2, as shown in FIG. 4. Therefore, alignment of a part of the liquid crystal tends to be inclined, so that the refractive index to be sensed by the incident light beam becomes closer to the refractive index of the planar concave lens, thereby making the amount of variation of the optical property smaller. Furthermore, there is a disadvantage in that distribution of the variation amount of the optical property depends on the position with respect to the lens.
Further, since the transparent electrode 2 is formed on the surface of the planar concave lens 1, when the voltage is applied, an electric field perpendicular to its surface is established in the vicinity of the transparent electrode 2 so that the liquid crystal 4 may be aligned perpendicularly to the surface thereof. As a result, there arises an inclination of the alignment of a part of the liquid crystal 4 to form a region where the refractive index sensed by the incident light beam is significantly different from the refractive index of the planar concave lens 1. Thus, the incident light beam which should pass through without any deflection substantially, is locally deflected.
Furthermore, in the case where the surface configuration of the planar concave lens 1 is more complicated, particularly when it has deep grooves or sharp projections, it becomes difficult to uniformly form the transparent electrode, so that a circuit breakage or high resistance is liable to occur.
Additionally, in such case, an alignment process of the alignment layers for aligning the liquid crystal, such as a rubbing process and the like, becomes difficult. Further, the distance between the transparent electrodes varies at different positions as is clear from FIG. 1. Despite this fact, since an equal voltage is applied to all positions of the transparent electrodes, degradation of insulation, short circuits, etc. are liable to occur in a narrow region.
As set forth above, the conventional active optical device employing a material having a variable refractive index encounters various practical drawbacks or shortcoming in production and driving.